1. Field of the Invention
The present invention relates to a decompression device provided for facilitating start-up of an internal combustion engine such as an air-cooled two-cycle gasoline engine and the like, and more particularly to a decompression device which provided in a relatively small-sized and light engine with a recoil starter suitable for use as a power source incorporated in such a work machine as a portable or conveyable sprayer or bush cutter and which is operative to reduce pressure in the combustion chamber in cooperation with cranking operation effected by the recoil starter.
2. Description of the Prior Art
Generally, in the work machine such as a portable or conveyable sprayer or bush cutter, an air-cooled two-cycle gasoline engine with a recoil starter which is relatively easy to reduce in size and weight is used frequently as a power source of the work machine.
It has hitherto been well known as described in, for example, Japanese Patent Application Laid-open No. Sho 61-38162 or Japanese Utility Model Application Laid-open No. Hei 3-95078 to additionally provide a decompression device which reduces pressure in a combustion chamber of the engine in cooperation with cranking operation effected by the recoil starter in order to facilitate start-up of the engine with the recoil starter.
The additional provision of the decompression device is fulfilled frequently not only in the engine with recoil starter but also in an engine with self starter, with a view of reducing size and weight as well as power consumption of a motor serving as a power source of the self starter.
Essentially, this type of decompression device is comprised of a decompression passage through which a combustion chamber defined by the inner wall surface of a cylinder and the top surface of a piston in an engine body communicates with an external component such as, for example, an intake passage, an exhaust passage or scavenging port, a decompression valve inserted in the decompression passage, and valve operating means for opening the decompression valve upon start-up operation (cranking period).
In the decompression device provided in the engine with a recoil starter, the valve operating means is designed to open the decompression valve in cooperation with cranking operation effected by the recoil starter. More specifically, as disclosed in the aforementioned literatures, the recoil starter is provided with a rotation transmission member which, when a recoil drum is rotated by pulling out rushingly the fore end of a recoil rope wound about the recoil drum in a starter cover, is moved in the axial direction in cooperation with the rotation of the recoil drum to engage with a crank shaft so as to transmit torque of the recoil drum to the crank shaft under the engaged condition, whereby a decompression operating rod or the like constituting the valve operating means is laterally moved by utilizing the axial moving motion of the rotation transmission member to rotate or push a spool or the like of the decompression valve so as to open the same, thereby causing the combustion chamber to communicate with the external component through the decompression passage.
When the combustion chamber and the external component are brought into communication with each other, a part of air-fuel mixture in the combustion chamber is admitted to the outside through the decompression passage to reduce pressure acting on the top surface of the piston, thereby ensuring that torque required to rotate the crank shaft can be reduced and pay-out of the recoil rope can be facilitated to make start-up of the engine easy.
In the recoil starter called the Bendix type as shown in the aforementioned two literatures, the rotation transmission member is rotated by torque applied to the recoil drum while the rotation transmission member being coaxially fitted in the recoil drum through spiral spline teeth to enable the rotative motion of the recoil drum to be utilized to axially move the rotation transmission member, and besides braking force is applied to oppose the rotation of the rotation transmission member in order to regulate co-rotation of the rotation transmission member along with the recoil drum, wherein a friction braking member (friction spring) for application of the braking force is used as the decompression operating rod or the like constituting the valve operating means.
In the prior art decompression device provided in the engine with the recoil starter as described previously, since as the rotation transmission member moves in the axial direction, the decompression operating rod or the like constituting the valve operating means is also moved in the same direction to operate the decompression valve, there is a need for the starter cover to be necessarily formed with an opening (window) having a certain width which allows the movement of the decompression operating rod or the like.
With the opening formed in the starter cover in this manner, external dusts and dirts will intrude into the starter cover through that opening. Inside the starter cover, sliding parts such as the recoil drum and rotation transmission member are arranged and when dusts and dirts intrude into these parts, operation of the sliding parts is caused to be defective and the defective operation of the recoil starter will induce defective operation of the decompression device cooperative with the recoil starter.
Especially, in the case where the conventional recoil starter and decompression device are additionally provided to the air-cooled engine frequently used for a work machine such as a sprayer or bush cutter, ambient air for cooling blows around the recoil starter or is drawn therein, with the result that foreign matter such as powder materials and pest control liquid or cut grasses mixed in the ambient air intrudes into the recoil starter through the opening or clogs therein and possibly, proper operation of the recoil starter and decompression device will be prevented.
The above defective operation of the recoil starter and decompression device invites significant problems that the engine faces difficulties in starting, the decompression valve is not closed completely to cause a decrease in engine output and the engine is forcibly started resulting in breakdown of the recoil starter and decompression device.
It will be envisaged that when dusts and dirts or foreign matter intrudes into the starter cover, they maybe removed; however it is very difficult to remove dusts and dirts in the starter cover externally thereof and there results a need of dismounting the starter cover and disassembling the recoil starter, requiring time-consuming and troublesome disassembling and assembling works, so that in effect the starter cover is frequently disregarded with dusts and dirts contained therein.
Further, the prior art decompression device is such that the decompression operating rod or the like constituting the valve operating means also serves as the friction braking member (friction spring) of the aforementioned recoil starter and is made of a spring material having certain resiliency, and therefore, in addition to the aforementioned problem of intrusion of dusts and dirts, inconvenience is liable to occur wherein when operating the decompression valve, the decompression operating rod tends to be flexed and bent by repulsive force exerting from the decompression valve and so the amount of opening of the decompression valve becomes deficient, leading to a failure to sufficiently reduce pressure inside the combustion chamber.